Improve wasm-c-api instantiation-time linking (#1902)

Add APIs to help prepare the imports for the wasm-c-api `wasm_instance_new`:
- wasm_importtype_is_linked
- wasm_runtime_is_import_func_linked
- wasm_runtime_is_import_global_linked
- wasm_extern_new_empty

For wasm-c-api, developer may use `wasm_module_imports` to get the import
types info, check whether an import func/global is linked with the above API,
and ignore the linking of an import func/global with `wasm_extern_new_empty`.

Sample `wasm-c-api-import` is added and document is updated.

core/iwasm/common/wasm_native.c

@@ -239,6 +239,10 @@ lookup_symbol(NativeSymbol *native_symbols, uint32 n_native_symbols,
     return NULL;
 }
 
+/**
+ * allow func_type and all outputs, like p_signature, p_attachment and
+ * p_call_conv_raw to be NULL
+ */
 void *
 wasm_native_resolve_symbol(const char *module_name, const char *field_name,
                            const WASMType *func_type, const char **p_signature,
@@ -264,10 +268,13 @@ wasm_native_resolve_symbol(const char *module_name, const char *field_name,
         node = node_next;
     }
 
+    if (!p_signature || !p_attachment || !p_call_conv_raw)
+        return func_ptr;
+
     if (func_ptr) {
         if (signature && signature[0] != '\0') {
             /* signature is not empty, check its format */
-            if (!check_symbol_signature(func_type, signature)) {
+            if (!func_type || !check_symbol_signature(func_type, signature)) {
 #if WASM_ENABLE_WAMR_COMPILER == 0
                 /* Output warning except running aot compiler */
                 LOG_WARNING("failed to check signature '%s' and resolve "